Subjecting bauxite to the Bayer process produces a pregnant caustic liquor containing dissolved alumina and a bauxite residue slurry. In order to extract alumina from the pregnant liquor, it is first subjected to solid liquid separation steps to separate the bauxite residue. The alumina-bearing liquor is then treated to precipitate aluminium trihydroxide that is subsequently refined to produce alumina, which is in turn processed into aluminium metal.
The separated bauxite residue is often in the form of a slurry that contains a significant volume of caustic liquor. The slurry is delivered to storage ponds where the bauxite residue solids settle out of the liquor to provide a “red mud” and a supernatant liquor. Environmentally safe disposal of the red mud is problematic because it contains naturally occurring contaminants, such as heavy metals, in concentrations greater than naturally occurring concentrations. The same applies to an extent to the supernatant liquor, but it also has the added problem of being caustic.
While options are available for recycling part of the supernatant liquor back to the refinery process, this capacity may be constrained by other process limitations within the refinery. This excess supernatant liquor combined with unfavourable climatic conditions that may exist, result in an ever increasing requirement for the storage of supernatant liquor. In the absence of suitable treatment processes, many bauxite treatment plants simply store Bayer process waste in ponds without treatment to reduce its environmental impact.
There is a need, therefore, to provide a process that is capable of treating considerable quantities of Bayer process waste and, in particular, the supernatant liquor.
It is preferable for the process to be able to reduce current reserves of supernatant liquor held in storage ponds and/or to treat the Bayer process waste at about the same rate that it is produced.